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Laying the coastal viaduct piers at sea

21 novembre 2016

Download the PDF : Laying the Coastal Viaduct bridge piers at sea

1/ Building the viaduct :

The 5400m offshore viaduct of the the NCR is being constructed using prefabricated reinforced concrete sections.

2 production sites have been operational since last year :

  • The first one in Port Est, with direct access to the sea, for prefabrication of the bridge piers.
  • The second in the Rear Port Zone for prefabrication of the voussoirs for the deck.

Once prefabricated, the sections are brought to the site using specific means of transport :

  • The bridge piers are brought in and installed by the ‘Zourite’ barge. Each pier is composed of two parts :

- the upper section, called the pier head, consists of a part of the bridge pier section of varying height and the column section upon which the deck will come to rest.
- the lower section, called the baseplate, consists of the base section which will rest on the seabed, and a part of the bridge pier section of varying height which will reach about 3 m above sea level.

The total height of the bridge piers varies from 24 to 38m. The pier baseplates rest between 12 and 15m below sea level, between 3 and 8 m beneath the ocean floor.

  • The voussoirs for the deck, weighing up to 670 tons, are loaded onto trucks (high capacity multi-wheeled trolleys) at the ZAP (industrial zone), and then transported along a specially organised road. The sections on supports go to Port Est, and the other elements to Grande Chaloupe. The sections on supports (mega bridge pier voussoirs, abutment voussoirs ), which are very heavy (up to 2400t), are transported and put in place on site by ‘Zourite’ the mega-barge), while the other elements are transported to the launcher station to be assembled (the launcher is the large beam currently being assembled at the eastern tip of Port Est).

The height of the deck voussoirs to be transported varies from 3.80 to 7.30m.

The prefabrication method of construction for the structure presents the following advantages :

  • On the one hand, it ensures a maximum amount of work is carried out ashore on the 2 prefabrication sites, in optimal conditions of work and safety,
  • On the other hand, it minimizes operations at sea, which can be very constraining, notably with respect to meteorological conditions. The possibility of working or not depends on predetermined criteria such as wave height, wind speed, type of activity, equipment used, etc.

This method requires considerable means to transport the prefabricated sections (trucks, launcher and ‘Zourite’).

2/ Laying a bridge pier :

The 48 bridge piers of the viaduct will be prefabricated at Port Est and then transported and installed on-site using ‘Zourite’.

‘Zourite’ is a 107m long by 44m wide barge, equipped with a gantry capable of carrying weights of up to 4800t. It is self-propelled and, thanks to its 8 legs, can be stationed at sea to install each bridge pier. It is also equipped with a concrete mixer, built specially for the project.

For each bridge pier, the construction sequence is as follows :

  • Prior to installation, a hole is dug at sea in the exact spot of its future position, in order to identify a terrain with the sufficient mechanical characteristics. The bottom of the hole is covered with a layer of 1m thick granular foundation in order to have a base which is as regular as possible ;
  • The baseplate (lower part of the bridge pier) is then transported onto ‘Zourite’ from the prefabrication plant in Port Est ;
  • ‘Zourite’ carries it across and sets down the baseplate. It is positioned very precisely, thanks to both the gantry and to the adjustment systems fixed on the bridge pier. The baseplate is positioned 40 cm above the granular foundation, to allow grouting between the underside of the base and the top of the granular layer. This grout makes it possible to have a perfect contact between the base and the ground ;

- Zourite then returns to Port Est to load the bridge pier head (upper section of the bridge pier), as well as the deck section to be loaded on top of the bridge pier (mega bridge pier voussoir or abutment voussoir, as appropriate) ;

  • A reinforced concrete central section between the two parts of the bridge pier is then constructed to fix the two elements, hence the need for a concrete mixer on board. The upper section of the deck is fixed to the bridge pier by means of pre-stressed cables.

After this, the launcher rests on top of the elements already in place at sea in order to assemble the deck voussoirs. These segments will be transported on land along the existing coastal road diverted to one side.